This invention relates to a method for controlling haze in an article comprising a polymer composition. More particularly the method relates to controlling haze in articles comprising polymer compositions, which compositions comprise an alkali metal halide.
An almost infinite variety of articles comprising polymeric materials form an integral part of modern commerce and technology. The utility of many such articles depends upon the transparent nature of the polymer composition from which the article is fabricated. In many applications requiring a high degree of optical clarity, the haze level exhibited by the article cannot exceed a certain threshold level. Certain polymer compositions exhibit a very high level of transparency and low haze values, for example, polycarbonate. However, other physical properties of such materials, for example glass transition temperature, make them unsuitable for use in many applications requiring both a high level of optical clarity and substantial resistance to the effects of heat. Many polymer compositions are available which possess outstanding heat resistance, but which are prone to afford articles fabricated from said polymer compositions which exhibit an unacceptable level of haze.
In some instances, the level of haze exhibited by an article comprising a polymer composition is found to be dependent upon the method by which the polymer composition itself is prepared. For example, articles fabricated using polyetherimide polymer compositions exhibit higher or lower levels of haze depending on the method used to prepare the constituent polyetherimide. Polyetherimide compositions are commercially attractive materials due to their combination of high heat performance, good mechanical properties, chemical resistance and ease of processing. Commercial Ultem® polyetherimide compositions find applications in dishware, film, silicon wafer carriers, and like applications, wherein, low haze is a critical quality factor. Typical haze levels of commercial Ultem® polyetherimide compositions are found to be below 2%.
As noted, however, the haze levels exhibited by articles fabricated from polyetherimide compositions are dependent on the process followed to prepare them. Currently, commercially available polyetherimide compositions are produced using a “nitro-displacement” process to generate bisphenol-A dianhydride (BPADA) in a series of steps starting from a mixture of 3-nitro- and 4-nitro-N-methylphthalimide. The polyetherimide composition is then prepared in a condensation polymerization of the BPADA with a diamine, such as meta-phenylenediamine. In an alternate approach, the diamine is first reacted with chlorophthalic anhydride to form a bis-chlorophthalimide, which is then reacted with the alkali metal salt of a bisphenol (e.g. bisphenol A disodium salt) in a “chloro-displacement” polymerization process. In contrast to the nitro-displacement process, the chloro-displacement process appears to have advantages with respect to both process simplicity and compositional flexibility. However, articles fabricated using polyetherimide prepared using the “chloro-displacement” process were found unaccountably to exhibit significantly higher haze levels than articles fabricated using polyetherimide made by the “nitro-displacement” process. The higher haze levels observed in articles fabricated using polyetherimide prepared using “chloro-displacement” polymerization detracts significantly from the commercial attractiveness of the “chloro-displacement” process technology.
Hence, there exists a need to provide a method for controlling haze in an article comprising polymer compositions, and particularly in articles comprising polymer compositions prepared using methods allied to the chloro-displacement process used to prepare polyetherimide compositions.